The present invention relates generally to color television receivers and, more particularly, to a color television receiver including an interface system adapted for selectively enabling the receiver for operation in a first mode wherein a color video image is conventionally reproduced in accordance with a received television signal or, alternatively, in a second mode wherein an alpha-numeric type display is produced on all or part of the screen in a selected color.
The use of color television receivers for providing on-screen displays of alpha-numeric type information, the term alpha-numeric type information as used herein being intended to comprehend both alpha-numeric characters as well as graphics, is generally known in the art, one relatively common example thereof being the on-screen display of channel number and time information. U.S. Pat. No. 4,081,797 to Olson discloses a system of this type wherein time and channel number information developed by a character generator is suitably multiplexed for developing a video signal which is used to drive the cathode ray tube (CRT) of a color television receiver. Another circuit designed for performing a similar function is disclosed in U.S. Pat. No. 3,812,285 to Miyata et al.
In addition to the on-screen display of time and channel number information, great interest has recently been expressed in other forms of on-screen alpha-numeric type displays including captioning for the deaf, teletext and viewdata information. In captioning for the deaf, as in the case of channel number and time displays, the alpha-numeric display is superimposed over a portion of the video image reproduced on the CRT screen in response to a received television signal whereas, in the case of teletext and viewdata information, the entire CRT screen is typically used to reproduce the alpha-numeric characters. In the past, the alpha-numeric information intended for display on the CRT screen has generally been inserted in the luminance signal path of the receiver so that only corresponding white images could be reproduced. As disclosed in U.S. Pat. No. 4,135,182 to Bell et al, it is also known to convert an alpha-numeric signal to a monochrome NTSC television signal which may then be coupled directly to the antenna terminals of a television receiver for reproducing the alpha-numerics in terms of a black and white image. U.S. Pat. No. 4,218,698 to Bart et al discloses a system wherein alpha-numeric type information signals are coupled to the R, G, B inputs of the video output stage of a color television receiver in place of the corresponding video signals developed in response to a received television signal, but this system uses a plurality transmission gates which are relatively expensive and therefore does not provide a circuit desirable for use in a mass produced consumer product.
It is therefore a basic object of the present invention to provide a novel circuit for facilitating the display of alpha-numeric type information on the screen of a color television receiver. It is a more specific object of the invention to provide a relatively inexpensive circuit capable of providing complete or partial on-screen displays, in a selected color, of alpha-numeric type information derived from various different sources including, but not limited to, channel number and time generators and teletext and viewdata decoders.